Welcome SheDee:
First there needs to be a consious decision; keep the tree or keep the garage floor.
If you choose to keep the tree, the garage needs to be torn down and removed. However, if you choose the garage with the floor and wall, its time to cut the tree, salt the stump for a complete kill, allow the stump to dry out for a year and set it on fire. The stump will burn away and the roots will continue to smolder for a week or two but it will eventually ease the strain against the floor and solve the problem.
Glenn